A theoretical analysis of the high frequency characteristics for an all-metal slow-wave system (SWS), i.e., a sine waveguide, is performed in this paper. The continuous profile of the cosine groove is approximated by a series of connected rectangular steps, and the SWS is divided into several regions reasonably, then the expressions of the field in all the regions are given. The dispersion equation and coupling impedance formula of the SWS are obtained by using the field matching theory combined with the matching of the admittance. The high frequency characteristics for a sine waveguide used in a 220 GHz TWT are discussed, and the calculation results are in good agreement with those obtained from three-dimensional electromagnetic simulation software CST-MWS. The effects of the geometrical parameters on high frequency characteristic of the sine waveguide are also investigated in detail.